Fixed some phpstan errors

src/WebSocketMiddleware.php

@@ -64,7 +64,8 @@ class WebSocketMiddleware implements EventEmitterInterface
             $this->emit(self::EVENT_CONNECTION, [ $websocket ]);
         //});
 
-        // TODO would it be possible or rather useful for the 'connection' event to set additional response headers to be sent here?
+        // TODO would it be possible for the 'connection' event to set additional response headers to be sent here?
+        // For example, to send back Sec-WebSocket-Protocol header.
         return new Response(
             Response::STATUS_SWITCHING_PROTOCOLS,
             array(